DEPARTMENT OF THE INTERIOR 

UNITED STATES GEOLOGICAL SURVEY 

GEORGE OTIS SMITH, Director 

Bulletin 530—F 


ALUNITE IN THE SAN CRISTOBAL 
QUADRANGLE, COLORADO 


BY AfA 

ESPER S. LARSEN 


ADVANCE CHAPTER FROM BULLETIN 530, “CONTRIBUTIONS TO 
ECONOMIC GEOLOGY, 1911, PART I” 


WASHINGTON 

GOVERNMENT PRINTING OFFICE 

1912 














DEPARTMENT OF THE INTERIOR 

UNITED STATES GEOLOGICAL SURVEY 

D 

* 

GEORGE OTIS SMITH, Director 


Bulletin 530— F 


ALUNITE IN THE SAN CRISTOBAL 
QUADRANGLE, COLORADO 


BY 

ESPER S. LARSEN 


ADVANCE CHAPTER FROM BULLETIN 530, “CONTRIBUTIONS TO 
ECONOMIC GEOLOGY, 1911, PART I” 




WASHINGTON 

GOVERNMENT PRINTING OFFICE 

1912 














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MAY 20 1S15 


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ALUNITE IN THE SAN CRISTOBAL QUADRANGLE, 

COLORADO. 


By Esper S. Larsen. 


INTRODUCTION. 

In the course of the geologic mapping of the San Cristobal quad¬ 
rangle, Colorado, under the direction of Dr. Whitman Cross, the 
writer examined a number of large areas of altered rock and found 
that alunite is a common and in places abundant mineral in the 
alteration product. The recent development of the potash industry, 
whereby alunite has become recognized as a possible source of potash, 
has given additional importance to occurrences of this mineral, and 
it is the purpose of this paper to discuss briefly the known alunite 
deposits in the San Cristobal quadrangle. 

GEOLOGY. 

The greater part of the quadrangle is occupied by volcanic rocks 
of Tertiary age, and these rocks extend for many miles to the north, 
east, and west. They range in composition from rhyolite to basalt, 
but even the basaltic rocks commonly contain a considerable amount 
of potash feldspars. Lava flows form most of the material, but the 
area contains breccia beds and extensive intrusive bodies. In gen¬ 
eral the flows are flat and the structure is simple, but in places they 
are much faulted and tilted. 

ALUNITE ROCKS. 

Areas of altered rock a mile or more across are common in the 
volcanic region. Some are elongated or lenslike and are associated 
with fractures or vein fillings, and in places, as at Carson, these 
narrow altered zones are very abundant; others are more irregular 
in form. The altered rock is nearly always bleached white, and it 
is commonly softer than the unaltered rock. Many of these white 
outcrops are conspicuous and can be recognized at a considerable 
distance. In some parts of the quadrangle the alteration has given 
rise to kaolinite or to sericite, but from the writer’s observations 
these types of alteration are not so extensive as alunitization, which 
has been observed in basaltic as well as in rhyolitic rocks. 


45302—12 


3 




4 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1911, PART I. 

The alunitized rock is usually white and commonly contains 
rather prominent cavities. As a rule the original texture of the 
rock is partly preserved. The alunitized rock can generally be dis¬ 
tinguished from kaolinized or sericitized rock by its more compact, 
crystalline appearance and vitreous rather than earthy luster. Crys¬ 
tals and cleavage faces of alunite can be seen in the more coarsely 
crystalline rock, but most of the material is very fine grained and 
only pyrite can be recognized in the hand specimen. The thin sec¬ 
tions show chiefly quartz and alunite, with a little pyrite, and some 
specimens, especially those derived from rather basic rocks, contain 
kaolinite or bauxite. The alunite is in some places scattered through 
the altered rock, but much of that derived from the more basic rocks 
partly replaces the plagioclase or occurs scattered through the ground- 
mass. In a felsitic'rhyolite at the head of South River small crystals 
of alunite are scattered throughout the altered rock, which retains 
some of its original felsitic texture. 

OCCURRENCES OF ALUNITE. 

Head of Middle Fork of Piedra River .—In the basin at the head of 
the west branch of the Middle Fork of Piedra River is a large area 
of altered breccia. The fresh rock contains abundant tabular 
crystals of labradorite and some augite and hypersthene in a ground- 
mass of plagioclase and orthoclase. Quartz is generally present in 
the groundmass, which varies greatly in coarseness. The altered 
rock always contains abundant quartz and kaolinite or bauxite and 
usually a considerable amount of alunite. Pyrite is invariably 
present and gypsum is prominent locally. It is interesting to note 
that in the southeastern part of this area there are deposits of banded 
opal and sulphur rock and altered rock consisting of quartz, kaolinite, 
sulphur, and pyrite, but no alunite was observed associated with the 
sulphur type of alteration. This altered zone is in an area of intense 
faulting, and while no faulting was recognized in the immediate 
vicinity of the rocks showing alteration the two are probably related. 

Head of South River .—At the head of South River, just east of 
Piedra Peak, the rhyolite which overlies the andesitic complex men¬ 
tioned in the preceding paragraphs is intruded by a large body of 
diorite porphyry. About this intrusive the rhyolite is much altered 
and to the east of Piedra Peak the trail crosses a lar^e bodv of this 
altered rock. Some prospecting for gold has been done in it, but 
the mineralized rock seems to be the only ore. The fresh rock is a 
lluidal felsitic rhyolite made up of quartz and orthoclase, with a little 
biotite. Three specimens of the altered rock were collected near the 
prospects and found to consist of a fine-grained aggregate of quartz, 
alunite, and pyrite. 

A partial analysis of the typical altered rock was made by W. T. 
Schaller, with the results given below in column 1. The chemical 


ALUNITE IN SAN CRISTOBAL QUADRANGLE, COLt). 5 

composition of the fresh rock is not known, but the analysis of a 
similar rhyolite from California is given for comparison. A com¬ 
parison of the two analyses shows a relative loss of some A1 2 0 3; 
little change in the total iron and Si0 2 , a loss of over half the K,0, of 
more than half the Na 2 0, and of nearly all the MgO and CaO. There 
has clearly been an addition of S and S0 3 . The chemical analysis 
indicates that the rock contains about 69 per cent of quartz, 2 per 
cent of pyrite, and 29 per cent of alunite, and this agrees very well 
with the results obtained from a study of thin sections under the 
microscope. 


Analyses of alunitized rhyolite from head of South River and of a rhyolite from California 



1 

2 


69.24 

11.50 

1.34 

1.99 

.80 

74.65 

14.11 

/ 1.08 

\ .29 

4.59 
2.81 
.20 
.80 


TTonHtotal iron. 

x eO \ 

K 2 0. 

Na 2 0. 

MgO. 

CaO. 





1. Alunitized rhyolite from head of South River. Analysis by W. T. Schaller. 

2. Rhyolite from California. Analysis by W. F. Hillebrand. 


Carson .—At the mining camp of Carson the quartz latite and other 
rocks carry numerous irregular or elongated areas of bleached and 
altered rock showing more or less mineralization. In general the 
alteration consists of kaolinization and sericitization, but near some 
of the veins quartz-alunite rock was found. 1 

Slumgullion Gulch .—The Slumgullion mud flow is made up chiefly 
of andesitic rocks which were derived from the slopes above it. Much 
of the rock is greatly altered and somewhat mineralized, and to judge 
from the specimens collected much of the altered rock contains 
abundant alunite. The alunite occurs in rocks made up chiefly of 
opal, in which it lines cavities or is scattered through the opal. Alu- 
nite-quartz rocks were also collected. These rocks came from some 
of the large areas of altered rock that are so prominent about Slum¬ 
gullion Gulch. 

Red Mountain .—Red Mountain, a few miles southwest of Lake 
City, is formed of great flows of quartz latite. The rock is character¬ 
ized by large crystals of feldspar, some quartz, biotite, and augite in 
an aphanitic groundmass. The large feldspars are in part ortho- 
clase, in part andesine or microperthite. The groundmass is made 
up of quartz and orthoclase. The rock is rather rich in alkalies. 
Locally it has been much altered, and the largest body of altered rock 
covers an area of over'a square mile, occupying the crest and slopes of 
Red Mountain. This rock is commonly stained red from iron, but 


.i Larsen, E. S., The economic geology of Carson cainp, Hinsdale County, Colo.: Bull. U. S. Geol. Survey 
No. 470, 1911, p. 33. 






















6 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1911, PART I. 

otherwise it is nearly white. Bodies of opal and quartz are abundant, 
but by far the greatest part of the mass is made up of quartz or opal 
and alunite, with some pyrite and a varying amount of kaolinite. 
The alunite and quartz are locally present in a very fine grained aggre¬ 
gate, but commonly the alunite crystals are collected in the original 
feldspar crystals and to a less extent scattered through the ground- 
mass. No analyses are available, but the microscopic study indi¬ 
cated a considerable amount of alunite. 

GENESIS. 

All the alunite occurrences described are clearlv the result of the 
alteration of igneous rocks. From a study of the thin sections and 
of the partial analyses of the South River rock it is believed that in 
the process of alunitization most of the silica of the rock remains as 
quartz and the iron as pyrite, and that most of the alumina goes into 
the alunite. The magnesia and lime are almost completely removed 
and the alkalies are much decreased, soda to a greater extent than 
potash. In the conversion of orthoclase to alunite and quartz with¬ 
out the removal of alumina two-thirds of the alkali is removed and 
there is an increase of about 14 per cent in weight and a somewhat 
smaller increase in volume. In the alunitization of a soda feldspar 
the increase in weight is nearly 20 per cent, and if some of the soda 
is replaced by potassium the change is even greater. Many of the 
alunitized rocks show porosity and other evidence that there was a 
decrease in volume on alunitization. This may be accounted for in 
part by the loss of the lime and magnesia but chieffy by the loss of 
alumina and possibly also of silica. 

The character of the alunite rock, which consists of quartz, alunite, 
and pyrite, and the absence of any oxidation products point to some 
other origin for the rock than descending oxidizing solutions. The 
same suggestion is offered by the great extent of the alunite rock in 
the areas on South River and Red Mountain, each of which covers 
approximately a square mile, and the exposure for a vertical distance 
of nearly 2,000 feet on Red Mountain, the uniformity of the alteration, 
and the lack of large veins or other sulphide bodies associated with the 
alunitization. Sulphates are known to occur as original constituents 
in some of the sulphide veins of the region. Indeed, in the vein of the 
Golden Fleece mine, on the east slope of Red Mountain, the mineral 
hinsdalite, which has the composition 2Pb0.3Al 2 0 3 .2S0 3 .P 2 Q 5 .6H 2 0 
and is closely related to alunite, is an abundant original constituent 
in a quartz sulphide vein. 1 

i Larsen, E. S., jr., and Schaller, W. T., Hinsdalite, a new mineral: Am. Jour. Sci., 4th ser., vol. 32,1911, 
p. 251. Irving, J. D., and Bancroft, Howland, Geology and ore deposits near Lake City, Colo.: Bull. U. S. 
Geol. Survey No. 478, 1911, p. 54. 




ALUNITE IN SAN CRISTOBAL QUADRANGLE, COLO. 


7 


The evidence suggests hot ascending solutions as the cause of the 
alunitization. The field relations point strongly to deep-seated hot 
sulphuric acid solutions without the aid of surface agents. However, 
in view of the fact that geologists do not generally admit the presence 
of such solutions, the evidence in the present case is not sufficient to 
justify the assumption of such a source for the alunitization in the 
San Cristobal quadrangle. The alternative source is the mingling of 
hot ascending solutions or gases carrying H 2 S and of surface oxidizing 
waters. 1 


1 For a discussion of the origin of alunite see Butler, B. S., and Gale, H. S., Alunite, a newly discovered 
deposit near Marysvale, Utah: Bull. U. S. Geol. Survey No. 511,1912, p. 21. 


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